Relative Humidity Influences Epicuticular Wax Load and Particulate Matter Accumulation on the Leaf Surface in <i>Asplenium nidus</i>

Abstract

Particulate matter (PM) has recently been considered one of the most harmful air pollutants to public health. Plants have been known to degrade and deposit particle pollutants with epicuticular wax (EW), and this capacity can be influenced by environmental conditions including relative humidity (RH). The present study examined the effects of RH on EW generation and PM deposition upon leaf surfaces within Asplenium nidus ‘Avis’. The plants were treated in growth chambers with two levels of RH (low: 30% - 40% and high: 80% - 90%) for a period of four weeks, and subsequently exposed to a 30 μg･m-3 concentration of TiO2 particles as a PM resource for 72 hours. The EW ultrastructure on the leaf surface was observed as the thin films type, which was not morphologically changed in the condition of low or high RH treatment. For four weeks of RH treatment, the fresh weight and leaf area per plant were not significant between low and high RH treatment, while dry weight was significantly higher in the high RH condition. We also found that greater amounts of EW per fresh weight, dry weight and leaf area were generated in high RH. However, the total amounts of PM deposition (surface PM + in-wax PM) of the plants were higher within the low RH treatment with a higher proportion of surface PM. In contrast the proportion of in-wax PM was 15% higher within the high RH. These results suggest that EW generation is affected by air humidity and that proportion of PM deposition in the EW layer were influenced by the amount of total wax load.